Lubricating oil compositions



United States Patent Ofi ice 2,971,912 Patented Feb. 14, 1961 2,911,912 LUBRICATING 01L COMPOSITIONS John Scotchford Elliott, and Eric Descamp Edwards,

London, England, assignors to Castrol Limited, a Britlsh company No Drawing. Filed May 13, 1957, Ser. No. 658,519

Claims priority, application Great Britain May 14, 1956 11 Claims. (Cl. 252-465) The present invention comprises improvements in or relating to lubricating compositions and relates particularly to fluid lubricants suitable for use over a wide range of temperatures and under a variety of conditions.

It is an object of the present invention to provide lubricating compositions suitable for use in internal combustion engines or other mechanisms where high temperatures are involved e.g. in the lubrication of gas turbines, especially the propeller turbine type where the lubricant is required to lubricate not only the bearings but also the reduction gearing, said lubricating compositions also being stable and fluid at low or very low temperatures e.g. down to 40 F. or even -80 F.

It has already been proposed to employ as lubricants (especially suited for use at low temperatures) certain diesters of aliphatic dicarboxylic acids with branchedchain aliphatic alcohols, a number of such esters being described in the Journal entitled Industrial and Engineering Chemistry, April 1947, p. 484 to p. 497.

Recent improvements in the design of gas turbine engines have increased the severity of the demands made upon the lubricant, which must have good oxidation resistance at high temperatures, e.g. of the order of 200' C., at the same time being non-corrosive towards metals used in the construction of the turbine engines, e.g. steel, copper, brass, magnesium, aluminium and cadmium plated steel. It must also possess a high load-carrying capacity, in order to provide adequate lubrication of the reduction gearing in the propeller-turbine type of engines.

In our United States patent application No. 456,063, now US. Patent No. 2,836,565, we have disclosed the fact that di(hydroxyaryl) thioethers are a class of thermally stable compounds which may be added to the esters above referred to in order to improve their oxidation resistance at high temperatures.

Other examples of effective antioxidants are phenm thiazine and some of its derivatives as described in the Journal entitled Industrial and Engineering Chemistry, December 1950, p. 2479 to p. 2489. I

Another class of antioxidants which are quite effective at high temperatures are the dialkyl selenides of relatively high molecular weight.

While these additives are all effective to a greater less degree in inhibiting the oxidation of ester-type synthetic lubricants, they sufier from the disadvantage of tending to break down at high temperatures with the consequent formation of products which are corrosive to cuprous metals and silver. Thus, for example, ester-type synthetic lubricants containing phenothiazine may, under certain conditions, produce at elevated temperatures an undesirable amount of tarnishing of copper and cuprous metals or silver, whilst di(hydroxyaryl) thioethers react with copper and cuprous metals to form copper derivatives which at very high temperatures, e.g. of the order of 280 C., tend to decompose with the formation of copper sulphides. As pointed out in United States patent application No. 456,063 the corrosive tendencies of di- (hydroxyaryl) thioethers towards cuprous metals may be reduced by the inclusion in the blend of certain basic aliphatic, alicyclic or heterocyclic amines, but these compounds are not completely elfective at the very high temperatures above referred to..

We have now found that by the inclusion in ester-type synthetic lubricants containing one or more antioxidants of the foregoing classes of a minor proportion of one or more heterocyclic compounds as hereinafter defined, effective protection of cuprousrnetals and silver can be achieved even at high temperatures.

Accordingly, the present invention provides a lubricating composition comprising a neutral ester or mixture of esters as hereinafter defined and a minor proportion of (a) a compound having antioxidant properties at elevated temperatures and containing the group C-S-C or -'CSeC- and (b) an organic heterocyclic compound containing two or three nitrogen atoms and one -C='C linkage in a five-membered heterocyclic ring.

The term neutral ester" used throughout the specification is to be understood as referring to esters having an acidity not exceeding 1 mg. KOH per gm.

The esters which are suitable for use as the lubricant base in accordance with the present invention are substantially neutral saturated organic carboxylic acid esters having a viscosity index of at least 100, a pour point or freezing point not exceeding --40 F., a boiling or decomposition temperature not less than 600 F. and a flash point not less than 350 F. It will be understood that mixtures of esters may be employed comprising individual esters which have freezing points above -40 F. and viscosity indices below 100, provided that the mixture itself has the desired properties.

The preferred esters have the general formula coon. -\COOR:

wherein Ris a saturated aliphatic or cycloaliphatic hydrocarbon radical having from 2 to 8 carbon atoms, and

C H; CH:

While esters having the above general formula are-pre-' ferred, it'is also within the scope of the invention to employ esters obtained by esterifying the dicarboxylic acids with a mixture of an alcohol and a glycol to give I complex esters having the general formula R OOCRCOOR OOCRCOOR wherein R, R and R 'are as defined above and 3 /OH Rs is a glycol or polyglycol having not more than about 12 carbon atoms.

Other types of complex esters which may be employed are prepared by esterifying a dicarboxylic acid (1 mol) with a glycol (2 mols) and a monocarboxylic acid (2 mols) or with 1 mol each of a glycol, a monocarboxylic acid and a monohydric alcohol or with 2 mols eachof a hydroxy-acid and an alcohol. Still other types of complex ester may be prepared by esterifying a glycol (1 mol) 3 l with a hydroxy-acid (2 mols) and a monocarboxylic acid (2 mols).

Other esters which may be suitable in certain circumstances are prepared by csterifying a glycol or polyglycol (1 mol) with 2 mols of a monocarboxylic acid, while still other esters may be obtained by completely esterifying with a suitable monocarboxylic acid polyhydric alco- 1101 such as trimcthylol propane or pentaerythrit'ol.

It is to be understood that in the foregoing formulae the radicals R R and R may, if desired, contain ether oxygen linkages. The mono-, diand polyhydric alcohols, and the monocarboxylic acids employed in the preparation of the complex esters may also contain ether oxygen linkages. The esters should, however, be free from elements other than carbon hydrogen and oxygen.

The preferred diesters are derived from suceinic, pyrotartaric, glutaric, adipic, pimelic, suberic, azelaic, sebacic and pinic acids, specific esters being:

Di( l-methyl-4-ethyl octyl) glutarate Di 2-ethyl hexyl) adipate Di(3-methyl butyl) azelate Di(Z-ethyl hexyl) azelate Di (2-ethyl hexyl) sebacate Di 3 :5:5-trimethylhexyl) sebacate Di(3:5:5-trimethylhexyl) azelate v 2 ethyl hexyl l-methyl heptyl sebacate Di 2-ethyl hexyl) pinate Di(C Oxo) adipate Bis(diethylene glycol monobutyl ether) adipate Di( l-methyll-ethyl octyl) sebacate Z-ethyl hexyl 3:5 :5 trimethyl hexyl sebacate.

Whereas the majority of esters falling within the foregoing class possess both a high viscosity index and low pour'point, certain of them e.g. di( 1:3 dimethyl butyl) adipate. di(3-methyl butyl) sebacate have relatively high freezing points (above 0 F.), and therefore would not normally be suitable for the purpose of this invention, except in admixture with other esters of lower freezing point.

It will be understood that different esters may be selected according to the conditions under which the lubricant is to be used. Thus for use at very high temperatures it will be preferred to employ the esters of high molecular weight and particularly the higher branchedchain di-esters of azelaic and sebacic acid.

The complex esters which may be employed are preferably prepared by esterifying 2 mols of the dicarboxylic acid, 2 mols of the branched-chain aliphatic or alkylsubstituted cycloaliphatic alcohol. and not more than 1 mol of glycol. Examples of glycols which may be used are ethylene, propylene and hexylcne glycols. Z-ethyl-l :3 hexanediol and the poly glycols, e.g. diethylerie, triethylene. and tetraethylene glycol.

When a monocarboxylic acid is used in preparing a complex ester this should be a straight or branched-chain aliphatic acid having from 2 to carbon atoms inclusive.

Specific examples of suitable complex esters which may be employed in accordance with the present invention are:

Ester prepared from ethylene glycol (1 mol), adipic acid (2 mols) and Z-ethyl hexanol (2 mols).

Ester prepared from tetraethylene glycol (1 mol), sebacic acid (2 mols), and 2-ethyl hexanol (2 mols).

Ester prepared from 2-ethyl-l:3 hexanediol (1 mol), sebacic acid (2 mols), and 2-ethyl hexanol (2 mols).

Ester prepared from diethylene glycol (1 mol), adipic acid (2 mols) and n-butanol (2 mols).

Ester prepared from polyglycol 200 (1 mol), sebacic acid (2 mols) and ethylene glycol mono (2ethyl butyl) ether (2 mols).

Ester prepared from sebacic acid 1 mol), tetraethylene glycol (2 mols) and caproic acid (2 mols).

Ester prepared from triethylene glycol (1 mol) adipic they should be soluble in the lubricant in the 4 acid (1 mol), n-c'aproic acid (1 mol) and Z-cthyl hcxanol (1 mol).

Ester prepared from scbacic acid (1 mol), lactic acid (2 mols) and n-butanol (2 mols).

Ester prepared from tetracthylene glycol (1 mol), lactic acid (2 mols) andv butyric acid (2 mols).

Examples of suitable glycol and polyhydric alcohol esters of monocarboxylic acids which may be employed are:

Triethylcne glycol di(2'-ethyl hcxanoatc) Hexanediol 1,6 di(2-ethyl hexanoatc) Trimethylol propane tri-n-octanoatc.

The antioxidants, additive (a), consist as already indicated of organic compounds having antioxidant properties at elevated temperatures and containing the group C-S-C- or --CSeC.-. Whilst a wide variety of compounds of this general type, e.g. the aliphatic sulphides, have certain antioxidant properties at elevated temperatures, many compounds have serious limitations at really high temperatures, e.g. of theorder of C., such as are encountered in the lubrication of gas turbine engines.-

The preferred antioxidants, which are efiective to a greater or less degree at high temperatures are of three different types:

(i) Di(hydroxyaryl) thioethers of the type described and exemplified in our United States patent application No. 456,063 and particularly di(hydroxyphenyl) thioethers in which the benzene nuclei are unsubstituted or substituted only by short-chain alkyl radicals.

(ii) Dibenzothiazincs having good antioxidant properties at elevated temperatures.

The dibenzothiazines are preferably dibenzo 1,4 thiazines. The benzo radicals may contain further substituents e.g. alkyl or aryl groups, or halogens.

The selenium analogues of these compounds may also be employed i.e. dibenzoselenazines.

Typical examples of compounds falling within this class are: 1

Phenothiazine (2,3,S,6 dibenzothiazinc) Phenoselcnazine Phenothiazone-3 Phenothiazine-S-oxide 10-10 diphenothiazine IO-benzyl phenothiazine 7-benzo (c) phenothiazine 3,7 difluorophenothiazine (iii) Dialkyl selenides having at least 20 carbon atoms e.g. dilauryl selenide and dicetyl selenide.

The antioxidants employed in the present invention should have a decomposition temperature of at least 200 C. and have adequate solubility in the esters in which they are to be dissolved.

The organic heterocyclic compounds,v additive (b),

are imidazoles, pvrazoles or 1,2,3 triazolcs. They contain respectively the groups C-NH where R=H, or an alkyl, alkenyl or aryl group.

The compounds selected should not boil or decompose below 200 C., and preferably not below 300 C., and

proportions in which they are intended to be employed.

Specific examples of such additives are:

Imidazole Benzimidazole Pyrazole Benzotriazole 2-methyl benzimidazole 3:5 dimethyl pyrazole Indazole The major proportion of the lubricating composition of the present invention consists of the esters hereinbeforedescribed. The antioxidants may be employed in proportions ranging from 0.05 percent to 5.0 percent, and preferably from 0.1 percent to 2.0 percent, by weight on the weight of the final composition. Generally the organic hetcrocyclic compounds will be employed in relatively small amounts e.g. from 0.01 percent to 2.0 percent, and

.exceed 20% by weight on the weight of the final composition.

In one form of the present invention the dicarboxylic addition of polymers of esters of acrylic or alkyLsubstituted acrylic acids, e.g. polymerised n-butyl or higher esters of methacrylic acid. Compositions of this type can be produced having sufiiciently high viscositics combined with very low pour-points.

The polymers are employed in proportions of from 1% to 15% preferably from 5% to by weight on the weight of the final composition;

Examples of polymers which may be employed are the polymerised n-hexyl, 3:5:5 trimethyl hexyl and lauryl esters of methacrylic acid, and various commercially available materials sold under the trade name Acryloid. It is preferred to use polymers which are not only freely soluble in the diester lubricants but also miscible with mineral oil.

Alternatively polyesters, e.g. polypropylene glycol sebacates of varying molecular weights may be used in place of the polymers of esters of acrylic or alkyl-substituted acrylic acids in order to increase the viscosity of the lubricating composition.

In a further form of the present invention there may be included in the lubricating composition certain amines in order to reduce the corrosion of cuprous metals and to impart to the blend increased stability towards oxidation. Examples of suitable amines are described in our United acid diesters of the type described are thickened by the Generally these amines will be employed in relatively small amounts, e.g. from 0.01 to 1.0 percent, and preferably from 0.05 to 0.5 percent, by weight on the weight of the final composition. a

In order to enhance the load-carrying capacity of the lubricating compositions of the present invention particularly when such compositions are to be used for gas I turbine lubrication there may be added to the compositions a proportion of one or more load-carrying additives. Thus there may be included in the composition a proportion of the neutral organic phosphate or phosphite esters disclosed in our United States patent application No. 483,824, now U.S..Patent No. 2,820,766.

Examples of such compounds include Tri(n-butyl) phosphate Tri(2-ethyl hexyl) phosphate Tri-o-tolyl phosphate Tri-rn-tolyl phosphate Tri(p-tertiary amyl phenyl) phosphate- Tri(methyl cyclohexyl) phosphate Tri(n-butyl phosphite) T ri phenyl phosphite Tritolyl phosphitc Tri cyclohexyl phosphite Di 3-carbomethoxy-4-hydroxyphenyl) thioether ditolyl phosphite The phosphate ester may be employed in an amount of from 0.5 percent to 10 percent, preferably from 1.0 percent to 5.0 percent, by weight on the weight of the final composition and the phosphite ester may be employed in an amount of from 0.1 percent to 5 .0 percent, preferably from 0.25 percent to 2.0 percent, by weight on the weight of the final composition.

Other additives may be included if desired, e.g. rust inhibitors such as very small amounts of metal petroleum sulphonates, or other known compounds capable of inhibiting rust formation in the presence of water.

Specific compositions which may be employed in accordance with the present invention are exemplified in the tables of test results which follow.

In one simple test method which was employed, strips of copper foil, approximately 2" x h", polished with carborundum powder, were placed in test-tubes which were partially filled with the oil under test, the copper strip being completely immersed. The test-tube was placed in a beaker of oil and heated in an oven.

In a first series of tests carried out according to this procedure, small proportions of different corrosion inhibitors were added to a blend consisting of di(3:5:5-

g trimethylhexyl) scbacate (D.N.S.) containing 1.0 percent of a technical grade of di(3-methyl-4-hydroxyphenyl),

States patent application No. 456,063. thioether. The results are summarised in Table 1.

.1 Table I Percent Appearance of cop r strip nlter- Inhibitor of De Inhibitor 4 hours 22 hours None Dari-I brownfblack,smooth Black flaky. Benzotriazole 0. 1 Brown deposit, on .light' Dark brown.

copper background. imidazolc 0 1 Brown/dark peacock Brown/very dark peacock. Bunzirnidazole 0 2 Very light peacock, light; Light peacock, medium brown deposit. brown deposit. Pyramlc 0.3 Light brown, white pow- Dark brown.

- dory deposit. 3:5 Dimethyl pyrazole 0.5 Light stain Smooth brown. Indamle 0. 3 Heavy greenish brown pow- Heavy dark brown deposit.

dery deposit. 2-Methyl benzimidazole.-. 0 2 Light peacocks Light acock with some dark rown deposit.

All inhibitors added to D.N.S.+1.0% di(3-methyl-4-hydroxy-phenyl) thioether. Temperature" 0. Duration of test 22 hours blends were tested, with and without the addition of It will be seen from Table I that all the compounds tested were effective to a greater or lesser extent, in inhibiting the attack of copper by technical di(3-methyl-4-. hydroxyphenyl) thioether, or by impurities contained therein, at 150 C.

A second series of tests wascarried out under the same conditions, but at 200 C. for 6 hours the results being summarised in Table II. In this series of tests, two

0.2% of benzotriazole.

Antioxidants of the dibenzothiazine type, such as phenothiazine, possessed greater thermal stability than the di- (hydroxyaryl) thioethers and prqducedf no more than light staining of the copper under the foregoing test con ditions, even at 250' C. (6 hours). 1

It was observed, however, that if the'test strips were only partially immersed in the oil, considerable attack of the copper took place above the oil, and particularly 10 at the oil-air interface. This attack could be appreciably Test No. Composition oi blend Inhibitor Appearance of copper strip 457 D.O.S i P i "i""""l'i' o ynony me aeryae 9 3 pmphm Mum...- le brown i Em-Dlhydroxy-diphenyl tbioet er 3% Tri-m tolyl phosphate 1%t ,p-Dihydroxy diphenyl thi e er 88% Di(2-ethyl hexyi) adipate.---- 10% Poly propylene sebacate ll 2% Di(3-methyl-4-hydroxyphenyl) thioetbcr (technical) +0.l% Dicyclohcxylamine. 887 Di (ii-ethyl hexyl) adlpate.- 10% Poly propylene sebacate Present.-. Matt ooppor colour, with some white deposit.

Absent Dark brown, 10% black.

12 2%Di(3-methyl-4-hydroxyphenyl) Present--- Medium brown.

thioet her (technical). +01% Dicyclohexylamine Temperature Duration of lost 6 hours.

Inhibitor 0.2% of bcnzotn'azole.

In this table, DOS" represents di(2+t hy l hexyl) sebacate, thepolynonyl methacrylate" used was poly (3:5:5trimethyl hex l) methacrylate, and the polypropylene sebacate was a commercially available material 7 trademark) In Table II, comparison between Tests Nos. 9 and 11 indicates that the principal causes of copper staining by own as "Reoplex 100" ("Reoplex" is a registered reduced by the inhibitors of the present invention, as demonstrated in Table III.

added to the blend.

Table III Test N 0. Composition or blend Inhibitor Appearance of copper strip omp ex er 13 3% Polynonyl methacrylat None sgg ggggg sg at and 0.5% Phenothiazino (techn 14 omp ex Benzimidazole (0 2 Smooth peacock above oil 37 Pciynonyl methacrylate.

0.5 Phenothiaiine (technical) g g bmwn at inter omp ex ster [m v 1o 3% polynonylmpthacrylatenn Bcnzotriazolc (0.05 Sigfipl: iii-$731 iiallattgreagig with 0.5% Phenothiazme (technical)..- 94% O. 5% Mineral Oil A l6 0.25% Calcium petroleum suiphonate None Heavy dark brown flaky in mineral oil). deposit at interface and 0.75% 7-Benzo (c) pheuothiazine just above. 94% ON S 5% Mineral 011 A 17 0.25% Calcium petroleum sulphonote Z-Methyl benzimidazole Light brown smooth above (20% in mineral oil). (0.05%). oil. with dark line at 0.75% 7-Benzo (c) phenothiazir-ie interface. N o deposit.

Temperature. Duration of test Copper strips partially immezs di(hydroxyaryl) thioethers at temperatures of the order of 150 to 200 C. are impurities, the relatively pure compound in Test No. 9 giving only a light brown stain, whereas the compound used in Test No. 11 gave some blackening even in the presence of dicyclohexylamine.

In both cases however a manifest improvemcm was sented 2-ethyl hexyl 3:5:5 trimethyl hexylsebacate.

obtained by the introduction of bcnzotriazole.

In a similar test at 200 C. with a silver specimen, a solution of 1% of di(3-methyl-4-hydroxyphenyl) thioether in DOS. produced a dark tamishon the silver, which did not form when 0.2% of benzotriazole was 250 C. 6 hours In Table III, the complex ester was of the type obtained by esterifying a dicarboxylic acid (2 mols) with a branched-chain aliphatic alcohol (2 mols) and a glycol (1 mol), and was a material known as ,Paraplex AP-52 (Paraplex is ,a. registered trademark). "ONS .repre- Mineral oil A was a solvent-refined mineral oil of viscosity about 65 seconds Redwood I at F.

A further series of tests was now carried out employing the oxidation/corrosion test procedure described in '35 Ministry of Supply Material Specification No. D. Eng.

. 9 1m. 2487 (Issue No. 2, dated 1st January, 1954 paragraph 7-4. This test was a combined oxidation and corrosion test designed to measure the tendency of an oil to develop corrosive oxidation products at elevated temwherein R is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R, are radicals selected from the group consisting of peratures. The results are summarise d in Table IV. 5 branched-chain alkyl and alkyl-substituted cycloalkyl rad- Zahie IV {All tests were carried act ior 22 hours at 140 0.}

Cadmium plated steel test-piece with copper catalyst Copper test-piece with steel catalyst Test Ester or Ester Blend Additives Colour oi- Acidity Acidity No, Wt. increase Wt. Colour of increase change (mgs. change Specimen (mgs. (mg!) Specimen Catalyst KOH/ (mgs.) KOH/ gm.) g

18 Di(3:5:5 trlmethyl None 252. 5 corroded Light peacock. 10.0 +1.2 Peacock 9. 5

hexyl) sebacate. l9 do 0.5% Dl(3-methyl+ +0. 6 Light ycllow Light brown" 0. 40 +3. 1 0. 40

hydroxy phenyl) thioether. 0.5% Dl(3-methyl-4- an do ygggggg fl +0.8 Almost clean" Light peacock. Nil -0.s Peacock Nil 0.1% Benzimidazole.-- 47% Di(2-ethyl hexyl) sebacnte 21 47% Dl(3;5:5 trlmethyl 0.05% Calcium petro- 180 ight grey-n- Bright...--.-- 15.8 -l.4 do 11.3

"" hexyl) sebncate. leum sulphonate.

6% Polynonyl me lute 47%b%i(%-ethyl hexyl) se sea e 22 47% Di(3:5:5 trimethyl 0.5% Phenothlazine +0.2 Trace oi'light Dark peacock Nil +0.2 Dark peacock} Nil hexyl) sebacate (technical). peacock with black brown.

6% Polynonyl meth- Stain. patches.

acrylate 47%bDl(%-ethyl hexyl) Se 8C3 e l 0.57 Phenothiazine 2a.... 47%;! E9 "g (t chnical. -1. e do Peacock 0.22 +1. 4 Slight peacock 0.10

g fig gg j 0.2% Benzotrlazoleuu v colonlation.

acrylate D I) 5.091g Trfini-tolyl 24.-- l 2-ethyl hex p osp a e sgbacatei Y 1.0% Phenotmmme. +0.2 Light peacock. vti'gjarliaw 0.06 +1. 6 50%:a gxllack 0.06

(purified) quer, almost dark peacock black in background. 5.0% 'Iri-m-tolyl Flamphosphate do 1.0% Pigerdigthiazine N ll do Light golden. Nil +0.2 Light gold Nil pur e 0.1% Benzotriazole.

Tests Nos. 18 and 21 demonstrate the very high acidities developed by uninhibited dicsters and blends based on them, with consequent severe corrosion of the cadmiumplated steel specimens. Tests Nos. 19, 22 and 24 show how acidity rise and attack of cadmium-plated steel can be effectively controlled by the presence of antioxidants of the types described, although this is accompanied by an undesirable attack on the copper specimens and catalysts which is particularly severe with phenothiazine. Tests Nos. 20, 23 and 25 show that, by the inclusion in the blends of the inhibitors of the present invention, this attack of copper can be reduced to negligible proportions.

We claim:

1. A lubricating composition consisting essentially of a substantially neutral saturated organic carboxylic acid ester having a viscosity index of at least 100, a pour point not exceeding F., a boiling temperature not less than 600 F., and a flash point not less than 350 F., the said ester being selected from the group consisting of an ester having the general formula COO R1 GOOR:

wherein R is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R are radicals selected from the group consistingv of branched-chain alkyl and alkyl-substituted cycloalkyl radicals having at least four carbon atoms, and an ester having the general formula icals having at least 4 carbon atoms, and R is selected from the group consisting of saturated hydrocarbon radicals and polyglycol radicals having not more than 12 carbon atoms; from 0.05 to 5.0% by weight on the weight of the final composition of a compoundselected from the group consisting of di(hydroxy aryl) thioethers and dibenzothiazines having good antioxidant properties at elevated temperatures; and from 0.01 to 2.0% by weight on the weight 'of the final composition of at least one compound selected from the group consisting of imidazole, benzimidazole, pyrazole, benzotriazole and indazole and methyl derivatives of any of the foregoing wherein said methyl groups are substituents of the carbon atoms of the ring.

2. A lubricating composition as claimed in claim 1 wherein said composition contains 'from 1l5%' by weight on the weight of the final composition'of a polymer selected from the group consisting of polymers of esters of acrylic and alkyl-substituted acrylic acids.

3. A lubricating composition as claimed in claim 1 wherein said composition contains about 0.5 to about 10% by weight on the weight of the final composition of an additional compound selected from the group consisting of neutral organic phosphate and phosphite esters.

4. A lubricating composition as claimed in claim I wherein the ester has the formula wherein R is a radical selected from the group consisting -of saturated aliphatic and cyclo'aliphatic hydrocarbon n is an integer from 2 to 8.

6. A lubricating composition as claimed claim 1 wherein the ester has the formula R OOCRCOQR;OOCRCOOR whereinR is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R, are radicals selected from the group consisting of branched-chain alkyl and alkyl-substituted cycloalkyl radicals having at least four carbon atoms, and R; is seleced from the group consisting of saturated hydrocarbon radicals and polyglycol radicals having not more than 12 carbon atoms.

7. A lubricating composition as claimed in claim 1 wherein the ester is a compound selected from the group consisting of di(3-methyl butyl) azelate, di(2-ethyl hexyl) azelate, di(2-ethyl hexyl) sebacate, di(3:5:5-trimethylhexyl) sebacate, di(3:5:5 trimethylhexyl) azelate, di- (1-methyl-4-ethyl octyl) sebacate and 2-ethyl hexyl 3:5:5 trimethylhexyl sebacate.

8. A lubricating composition consisting essentially of a neutral ester having the formula cooni COOR:

wherein R is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R, are radicals selected from the group consisting of branched-chain alkyl and alkyl-substituted cycloalkyl radicals having at least four carbon atoms, from 0.1 to 2.0 percent by weight on the weight of the final composition of a dibenzothiazine having good antioxidant properties at elevated temperatures, and from 0.05 to 0.5 percent by weight on the weight of the final composition of at 5 least one compound selected from the group consisting of imidazole, benzimidazole, pyrazole, benzotriazole and indazole and methyl derivatives of any of the fore going wherein said methyl groups are substituents of the carbon atoms of the ring.

9. A lubricating composition consisting essentially of a neutral ester havingthe formula COOR! \COORI wherein R is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R are radicals selected from the group consisting of branched-chain alkyl and alkyl-substituted cycloalkyl radicals having at least four carbon atoms, from 0.1 to 2.0 percent by weight on the weight of the final composition of a di(hydroxyaryl)thioether, and from 0.05 to 0.5 percent by weight on the weight of the final composition of at least one compound selected from the group consisting of imidazole, benzimidazole, pyrazole, benzotriazole and indazole and methyl derivatives of any of the foregoing wherein said methyl groups are substituents of the carbon atoms of the ring.

10. A lubricating composition consisting essentially of a neutral ester having the formula wherein R is a radical selected from the group consisting of saturated aliphatic and cycloaliphatic hydrocarbon radicals having from 2 to 8 carbon atoms, and R and R are radicals selected from the group consisting of branched-chain alkyl and alkyl-substituted cycloalkyl radicals having at least four carbon atoms, from 0.1 to 2.0 percent by weight on the weight of the final com position'of phenothiazine, and from 0.05 to 0.5 percent by weight on the weight of the final composition of benv zotriazole.

11. A lubricating composition consisting essentially ofat least one ester selected from the group consisting of di(2-ethyl hexyl) sebacate and di(3:5:5 trimethylhexyl) sebacate, from 0.1 to 2.0 percent by weight of phenothiazine and from 0.05 to 0.5 percent by weight of benzotriazole.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Lubrication Eng., August 1952, pages 177-179.

Ind. and Eng. Chem., April 1947, vol. 39, No. 4, pages 491-497. 1

Ind. and Eng. Chem, December 1950, vol. 42, No. 12, pages 2479-2489.

Ind. and Eng. Chem, August 1953, vol. 45, No. 8,

pages 1766-1769. 

1. A LUBRICATING COMPOSITION CONSISTING ESSENTIALLY OF A SUBSTANTIALLY NEUTRAL SATURATED ORGANIC CARBOXYLIC ACID ESTER HAVING A VISCOSITY INDEX OF AT LEAST 100, A POUR POINT NOT EXCEEDING -40*F., A BOILING TEMPERATURE NOT LESS THAN 600*F., AND A FLASH POINT NOT LESS THAN 350* F., THE SAID ESTER BEING SELECTED FROM THE GROUP CONSISTING OF AN ESTER HAVING THE GENERAL FORMULA
 3. A LUBRICATING COMPOSITION AS CLAIMED IN CLAIM 1 WHEREIN SAID COMPOSITION CONTAINS ABOUT 0.5 TO ABOUT 10% BY WEIGHT ON THE WEIGHT OF THE FINAL COMPOSITION OF AN ADDITIONAL COMPOUND SELECTED FROM THE GROUP CONSISTING OF NEUTRAL ORGANIC PHOSPHATE AND PHOSPHITE ESTERS. 